This invention relates to multi-stage laser amplifier systems, and more particulary to a method and apparatus for providing interference-free multi-pass operation of laser amplifier systems.
Multi-stage amplification of laser beams is necessary in many applications to achieve the desired system gain. This is because each laser amplifier is limited in efficiency and power handling capability, so that single stage gain is often much less than desired. Numerous multi-stage amplifier systems have been devised to provide system gain requirements. One system simply comprises a series of individual amplifier stages, separately pumped, that are sequentially arranged to provide cascade amplification. Because this system uses multiple amplifier stages, pumping stages and optics, a large chain of amplifiers can become very expensive, unreliable and difficult to align.
Another system is a modification of the sequentially arranged multi-stage amplifier system, in which the amplifiers of the system are arranged in parallel to save space. The optical path of each amplifier stage output is folded over to feed the input of the following amplifier stage mounted next to it. This system is still very expensive, unreliable and difficult to align. However, this system shows how the path of the amplified beam can be folded a number of times to achieve multi-stage amplification.
A true multi-pass amplifier system folds the path of the amplified beam a multiple number of times through the same laser gain medium, allowing a single amplifier stage and pump stage to be used. This design saves space, reduces expense, increases reliability and makes alignment easier. However, in the simplest multi-pass amplifier of this type, the amplified beam overlaps itself during each pass.
An input pulse of x nanoseconds is 30 cm * x in length when it passes through a gain medium such as titanium-doped sapphire. Thus, in most amplifiers of this type, the input is present in the amplifier at the same time as subsequent passes of the amplified beam, so that interference results. Output beam quality is significantly diminished as a consequence. A variation of this type of amplifier is known as a "bow tie" amplifier, wherein the amplified beam is folded back through the same laser gain medium at different angles for each pass, so that the region of interference is limited to the vicinity of the intersection of the passes of the beam through the laser gain medium.
A multi-pass amplifier can be arranged to fold each pass of the amplified beam through the same laser gain medium in such a way that none of the passes of the beam intersect each other. The folds look much like bicycle spokes in arrangement. This type of multi-pass amplifier is described herein as a "bicycle spoke" amplifier. This design has all the advantages of the other multi-pass amplifiers, plus the additional advantage of freedom of interference between passes of the amplified beam that is folded back through the laser gain medium.
The multi-pass amplifiers according to the prior art that have a bicycle spoke configuration have one serious shortcoming, however. The overlap of the several passes of the amplified beam with the pump beam is very inefficient, because the profile of the region of the laser gain medium traversed by the passes of the amplified beam and the pump beam are so different. The pump beam has a substantially circular profile, whereas the passes of the amplified beam combine into a roughly rounded rectangular profile. Thus, a great deal of pump energy is wasted, and the gain of the amplifier is significantly restricted as a consequence.